The antenna for electromagnetic transduction (transmitting and or receiving) is often a critical part of sensing and communication systems. In the past, simple antennas could often meet the relatively narrowband requirements of radio communications. The increased bandwidth requirements of television placed a premium on bandwidth, both to provide cost-effective solutions for the need to cover VHF bands such as 54 to 88 megahertz (MHZ), corresponding to television channels 2 through 6 and 174 to 216 MHz (channels 7 through 13). Six-megahertz Channels within these bands were often received by simple dipole antennas, with tuning to improve the response.
The introduction of color television introduced the need for phase control over each television channel in order to avoid color distortion. Various new types of broadband antennas were introduced, such as the log-periodic monopole and the equiangular spiral antenna, which, at least in theory, could have infinite bandwidths. Physical limitations, such as the need for extremely precise fabrication of the small, high-frequency responsive portions, prevented the practical bandwidths of such antennas from exceeding about 10:1. Such antennas were, and continue to be, used for various forms of surveillance and monitoring.
Many applications for which antennas are used require scanning of the antenna beam more rapidly than physical movement of the antenna allows, and may require relatively high directivity. Those skilled in the antenna arts know that array antennas are useful for such applications. A wide variety of array antennas is known. Basically, an array antenna is a one or two-dimensional arrangement of a plurality of antenna elements. Sometimes two-dimensional arrays may be nonplanar, in which case the array has three-dimensional aspects. Each antenna element (or sometimes subgroups of antenna elements) of an array is “fed” with a common signal, which may be individually phase-shifted or adjusted in amplitude to accomplish beam scanning, as known in the art.
The antenna is basically a transducer which transduces electromagnetic signals between guided waves (signals flowing in a transmission line) and unguided waves (radiated signals). For this reason, the operation of an antenna in a transmission mode of operation is basically the same as its operation in a receiving mode, and has the same characteristics. Some of the terms used in the antenna arts originated at a time when the mechanisms by which antennas operate were not well understood. For example, the antenna “feed” point or terminals was defined at a time when the transmitting function was of prime importance, and is the terminal(s) at which a signal to be transmitted (transduced to an unguided wave) is applied. Only later was it fully understood that the feed point of a “transmitting” antenna is functionally identical to the received signal port of the same antenna when receiving signals. Thus, one general term for an antenna usable for either transmission or reception is “radiator” or “radiating element.” The “beam” of an antenna is a general term representing the radiated field and its characteristics. It is now understood that a given antenna has identical transmitting and receiving beams. A description of the operation of an antenna may be couched in terms of either transmission or reception, with the other mode of operation being understood as inherent therein.
The arraying of antenna elements to form array antennas introduced a host of new problems into the field. Among these problems is the interaction or mutual coupling among the elemental antennas, which can affect the apparent performance of the elemental antenna, and the need for a “beamformer” to combine the signals received by the individual antenna elements (reception mode) or divide the signal to be transmitted among the elemental antennas (transmission mode). The mutual coupling and the need for a beamformer having its own impedance characteristics makes for an intractable problem.
U.S. Pat. No. 4,782,346, issued Nov. 1, 1988 in the name of Sharma describes a horn-like finline antenna suitable for broadband operation in an array environment. The finline antenna described therein includes a dielectric plate with electrically conductive elements of the finline antenna printed or applied to each side of the dielectric. The dielectric plate(s) are fed by a rectangular waveguide.
Improved or alternative antennas are desired.